For Eri and VelociraptorBlade:

The Astrophysical Journal Letters is the overflow and monograph publication for the ApJ, and is about as legit as you can get in astrophysics. Once you are published in the ApJ you have ARRIVED!

Positrons are often the intermediate by-products of certain radioactive decay sequences and cascades, so they could be the result of energetic cosmic ray collisions with atoms in the upper atmosphere, creating all sorts of particle debris which are subsequently sorted out by the electromagnetic gradients up there. Eventually, these positrons would find electrons, and they would annihilate, but if the source process can keep up with the annihilation, an equilibrium of free positrons could exist in those high vacuum conditions where particle encounters are infrequent.

Depending on the concentration of these particles, an orbiting spacecraft could collect them (perhaps with an electromagnetic scoop like the input of a Bussard Ramjet) and the positrons combined with ordinary matter to generate energy in the form of high-energy gamma rays.

The technical problems of converting this radiation to energy, and then transmitting it somehow to the end-user are enormous, and it is not clear if there are enough positrons up there to make it feasible, but at least it violates no physical law and is, in principle, doable with contemporary technology.